Sulphur-containing fertilizers are in ever larger demand for compensating sulphur deficiencies in the soil. Conventionally, sulphur has been applied to the soil in the form of elemental sulphur, or as compounds such as ammonium sulphate, ammonium bisulphate, thiosulphates, sulphides or gypsum, or in combination with other fertilizer materials such as urea, for example as a sulphur-coated urea, as disclosed in U.S. Pat. No. 3,903,333 (Tennessee Valley Authority, 1975) and U.S. Pat. No. 5,599,374 (RLC Technologies LLC., 1997).
Sulphur-containing fertilizers, including fertilizers that contain elemental sulphur, are already known for a long time, the first patents on fertilizers containing elemental sulphur being issued more than 50 years ago. A sulphur-containing fertilizer solves the need to provide sulphur as a nutrient to plants. An agronomical benefit for using elemental sulphur is that a fertilizer comprising elemental sulphur can offer a higher nitrogen content in the fertilizer in the presence of a high sulphur concentration, e.g. over 42 weight % of nitrogen (N) over 8 weight % of sulphur (S) in a urea/sulphur fertilizer.
However, in the case of elemental sulphur, as such, it is not biologically available and needs to be converted to sulphates by bacteria in the soil and dissolved into water, available in the soil, in order to be of nutritional value to the plant. Therefore, other solutions have been found, such as the provision of urea-ammonium sulphate (UAS), in which the sulphur source is dissolvable into water and does not need a biological conversion.
Lately, new efforts have been devoted to the manufacture of urea-based fertilizers containing elemental sulphur, manufactured from a melt-mixture of urea-based base material and elemental sulphur.
U.S. Pat. No. 3,100,698 (Shell, 1963) discloses a fertilizer composition consisting essentially of co-melted and prilled urea and elemental sulphur. It is manufactured by mixing a liquid flow of fertilizer at a temperature of 141° C. and a liquid flow of elemental sulphur at a temperature of 127 to 142° C. using a pump, and it is prilled using a fan spray atomizing nozzle in a classical prilling tower. Vigorous stirring is necessary to avoid phase separation. Instead of employing a prilling tower, the product may be made by other techniques such as by granulating, spherodizing or flaking. The main disadvantage of the sulphur-urea product made according to this method is that the elemental sulphur does not oxidize rapidly enough to provide nutrient sulphur that is available early in the growing season and the sulphur becoming available only in the later stages of plant growth.
U.S. Pat. No. 4,330,319 (Cominco Ltd, 1982) discloses a process for making a urea-based fertilizer comprising elemental sulphur by mixing molten urea and molten elemental sulphur to obtain a molten mixture and solidifying the molten mixture to obtain a particulate urea-based fertilizer comprising elemental sulphur, passing the molten urea and the molten elemental sulphur through a mixing device (static mixer) at a temperature above the melting points of the urea and elemental sulphur in relative amounts, sufficient to produce said urea-based fertilizer comprising elemental sulphur, maintaining a pressure drop across said mixing device of at least about 200 kPa to form a homogenized melt of urea and elemental sulphur, and solidifying said homogenized melt in an inclined rotating granulation drum to obtain a homogeneous, solid, particulate urea-based fertilizer comprising elemental sulphur wherein the elemental sulphur phases have a size of smaller than about 100 μm. Essential in this process is the provision of a homogeneous melt by the use of a mixing tube with a “T” shape for joining the molten elemental sulphur flow with the molten urea flow, a melt that is subsequently homogenized in a mixer, then solidified into solid particles by a rotating drum. It is further disclosed that any one of a number of other methods can also be used, including prilling using a cooling gas in a tower, an inclined rotating pan or a fluidized bed.
It was found that a small elemental sulphur phase size was favourable for an efficient bacterial conversion into sulphates and that the phase size should be equal to or smaller than 100 μm, preferably equal to or smaller than 20 μm, for the bacterial conversion into sulphates to be fast. Hence, research has been performed to minimize the size of the elemental sulphur phases within the urea fertilizer particles by adding a surfactant.
WO03/106376 (Norsk Hydro, 2003) discloses the use of an additive, preferably a C6-C30 straight chain fatty acid, such as myristic acid, being temperature stable and amphoteric, to obtain a homogeneous mixed phase.
WO2014/009326 (Shell, 2014) discloses mixing a first flow comprising a liquid fertilizer with a second flow comprising liquid elemental sulphur in a mixing device in the presence of a multifunctional ionic surfactant to form an emulsion comprising elemental sulphur particles which are coated with a layer of the surfactant and dispersed in a fertilizer material that can be solidified.
ReSulf® is an example of a commercial product, sold by Yara International ASA, being a particulate urea-based fertilizer comprising small phases of elemental sulphur with a 42-9S composition, and produced from a micro-emulsified elemental sulphur in a liquid urea basis using a surfactant and solidified using a classical prilling technique.
Not only prills, but also pastilles of a urea-based fertilizer comprising elemental sulphur were produced by Yara International ASA (Oslo, Norway) with a 42-9S composition using a cooling belt (Sandvik, Stockholm, Sweden and in Nitrogen+Syngas 313, September-October 2011).
It would be advantageous to have an elemental sulphur-containing particulate urea-based fertilizer which not only has a high nutrient content and a rich N:S ratio from an agronomical point of view, but also contains elemental sulphur in a form and with a particle size that is more readily and quickly available as a plant nutrient. Such a fertilizer can be applied and can be effective early in the growing season or at other times.
All of the known methods to solve the aforementioned problem focus on the use of a homogenous mixed melt and/or the use of a surfactant to minimize elemental sulphur phase size.